This invention relates to optical pumps, and in particular, to a more efficient laser pump cavity which will more uniformly and more efficiently illuminate and stimulate a laser element to induce a population inversion in the laser element and an accompanying lasing condition.
One preferred embodiment pertains to the field of optically pumped lasers that use transverse or side pumping to excite a laser element with pump light traveling nominally perpendicularly to the axis of the laser output beam. The present invention relates to a device for redirecting and distributing radiant energy or light from a nominally cylindrical or line source to an object or target. A preferred embodiment is the use of this device as a pumping cavity for a laser. In that application the present invention relates to a device capable when stimulated of having a population inversion established therein and an accompanying lasing condition, and one or more pump sources positioned to pump radiant power into the sides of the lasing medium.
The most common shape of laser elements has been a cylindrical rod. Early dielectric solid state laser rods were transversely pumped by helical flashlamps wrapped around the laser rod. Subsequently linear lamps became widely used. These linear lamps have been positioned colinearly with the laser element and have been incorporated into laser pumping cavities of various designs, including close-wrapped, single elliptical, multiple elliptical, and cylindrical pump cavities. The reflecting walls have been on the inside of cavities or on the outside of transparent materials which, in part, support the laser element in the cavity.
Subsequently lasers and laser diodes began to replace flashlamps as pump sources. The light from a laser diode is more directional than the light from flashlamps, but is typically still highly (i.e. more than 50.degree.) divergent in one direction. Very short focal length lenses are commonly used to reduce the divergence of this light. An advantage of my invention is that the expense and complication of these lenses may be eliminated.
Laser diodes have been used alone or in groups to pump laser materials either longitudinally or transversely. In longitudinal pumping, the pump light travels generally coaxially with the laser beam. In transverse or side pumping, the pump light travels generally perpendicular to the axis of the laser beam. Often a monolithic linear array of laser diode emitters, called a laser diode bar, is used. These linear arrays are typically from three to twenty millimeters in length and may contain several hundred individual emitters. This invention is particularly well suited for use with laser diode bars, as will be described below. Stacks of these laser diode bars have also been used to create two dimensional arrays of laser diodes to pump slabs or larger diameter rods. Complicated reflector shapes have been used to couple the light from two-dimensional laser diode arrays to laser elements. An advantage of this invention is that only simple shapes are required.
More recently upconversion pumping has been employed. In upconversion pumping the pump light wavelength may be longer than the lasing wavelength. Two pumping steps are employed, but in some cases both steps use the same wavelength.
Often it is necessary to actively cool the laser element to remove the heat generated by the absorption of the pump light. It is generally necessary to also provide means for cooling the pump source and the cavity. The cavity structure is heated by absorption of the pump light and conduction from the pump source and laser element. Thus the design of a pumping cavity must consider methods of cooling the laser element, the pumping source and the cavity. Three general laser element cooling techniques have been used. The laser element can be radiatively cooled, immersed in a flowing cooling fluid, or attached to a heat sink which conductively cools the laser element. Unless a transparent heat sink is used, it can be difficult to achieve symmetrical pumping with conductive cooling as the heat sink interferes with the passage of the pumping light. In the case of liquid cooling, the liquid must be nominally transparent to the pumping light and not subject to severe photodegradation from the pumping light.
The efficiency and maximum power of many higher gain lasers may be improved by stifling parasitic oscillations and/or amplified spontaneous emission. As will become clear, this invention provides convenient and effective means for such stifling.
It is generally desirable that the pump light be absorbed symmetrically and completely by the laser element to maximize the efficiency of the laser. It may be desirable that the laser element be pumped more intensely along its central axis and with lower power about the periphery or circumference of the laser element. If the laser element does not absorb the pump light symmetrically, the laser element will be heated asymmetrically. This can lead to pump-induced beam steering and distortion of the laser modes. Much of the detailed design and experimental effort of the past has been devoted to developing practical pump cavities that insure efficient and uniform pump irradiation. In the case of laser diode bars, one way of achieving uniform pumping is to place several bars symmetrically around the laser rod. However, each bar must then operate at exactly the same power and wavelength. This has been achieved by controlling the temperature, hence the wavelength, and electrical power of each individual bar. Such individual controls add greatly to the cost and complexity of the laser. These controls are useful as they keep the wavelength of the diodes tuned to the maximum or proper absorption in the laser element. As the wavelength drifts off of the optimum value, the absorption in the laser element is altered. The alteration of absorption is usually a reduction in absorption. This may also reduce the efficiency of the laser. An advantage of this invention is that efficient and uniform pumping may be obtained over a wide range of absorption coefficient values in the laser element.
Great progress has been made in increasing the power available from an individual laser diode bar. As the power per laser diode bar increases, the number of bars needed to produce a laser of any given power decreases. With fewer bars, it becomes (using presently available pumping cavities) more difficult to achieve symmetric pumping. It is therefore one intent of this invention to make a laser pump cavity that can economically, efficiently and symmetrically pump a laser rod using only one, or a few laser diode bars. As will become clear, this invention will also work with flashlamps and other optical pump sources, as well as with laser elements other than laser rods.
Some lasers require two or more pumping wavelengths The additional wavelengths may be required to depopulate certain energy levels that are detrimental to efficient lasing, to provide the second step in two stage or upconversion lasers, or to provide two or more lasing wavelengths from a single lasing element. As will become clear, this invention is well suited to lasers requiring or benefiting from pump light of two or more wavelengths.
Still another advantage of this invention is that the pump source need not be contained within the reflecting walls of the pump cavity. The source may be external to the pump cavity with the pump light entering the pump cavity through a window or non-reflecting portion of the cavity wall. Thus the laser cavity need not be disturbed when the source must be replaced or adjusted.